Recently, as portable electronic instruments, such as portable computers, portable phones and camcorders, have been steadily developed so that they are downsized and lightened, lithium secondary batteries used as drive sources for the electronic instruments are also required to have high capacity, compact size, light weight and small thickness.
A lithium secondary battery comprises a cathode, an anode, a separator and an electrolyte. Each of the cathode and the anode comprises a collector and an electrode film, wherein the electrode film comprises an electrode active material, a conductive agent, and an electrode binder. The electrolyte infiltrates into the battery.
In general, quality of a lithium secondary battery largely depends on the electrodes, the electrolyte and other materials used for the battery. Among these factors, the content of the active material introduced into the electrode relates to the maximum value of available lithium ions. Hence, as the active material content increases, a battery has higher capacity. If a binder, which is one of the constitutional elements of a battery, shows excellent adhesion so that the amount of the binder needed for the battery can be reduced, it is possible to provide an electrode having an increased content of electrode active material. Therefore, there is a need for a binder having excellent adhesion.
Polyvinylidene fluoride homopolymer (PVDF), which is a commercially available electrode binder, has the advantages of excellent chemical and electrochemical stability. However, PVDF has disadvantages in that it should be used in the form of a binder composition after being dissolved into an organic solvent such as NMP (N-methyl-2-pyrrolidone), thereby causing an environmental problem, and it should be introduced into a battery in a relatively large amount in order to provide sufficient adhesion.
To solve the above problems, an attempt has been made to form a high-efficiency binder composition that uses water as a dispersion medium and is introduced into a battery in a decreased amount (Japanese Laid-Open Patent No. Hei4-51459). However, such binder composition is problematic in that it may cause degradation in the quality of a battery, particularly, rate characteristics and lifespan characteristics of a battery, compared to a battery using PVDF.